1. Field of the Invention
This invention relates to improved catalytic reaction systems and to methods for catalytic reaction of carbon containing compounds. In one specific aspect, the present invention relates to improving the catalytic conversion of gases in a catalytic converter utilizing short channel substrate.
2. Brief Description of the Related Art
Undesirable emissions from internal combustion engines, particularly those in automobiles, continue to be a major environmental problem in spite of significant reductions in these emissions resulting from the use of catalytic converters. A factor limiting the performance of catalytic converters is that the undesirable emissions are a concoction of pollutants, each pollutant placing an individualized requirement on the catalytic converter to effect its conversion. The conversion ability of today's catalytic converters is dependent upon the availability, or lack thereof, of oxygen. For example, the destruction of carbon monoxide and unburned hydrocarbons requires the presence of sufficient oxygen where as the destruction of nitrogen oxides is inhibited by oxygen. If all three pollutants are to be controlled, a mechanism is required to regulate the presence and quantity of oxygen within the catalytic converter.
Oxygen availability within the catalytic converter can be regulated by operating the engine such that the amount of oxygen delivered to the catalytic converter is controlled by regulating oxygen entering the engine relative to the fuel to be consumed, thereby regulating the oxygen within the engine exhaust gas stream. This control scheme requires that the engine be operated very close to stoichiometric fuel-air ratio if all three pollutants are to be controlled. Although the mean stoichiometry can be closely controlled, under today's driving conditions large variations can occur with typical control systems causing catalytic conversion inefficiencies and thus undesirable levels of pollution.
To refine the regulation of oxygen within the converter, substrate coatings utilized within the converter have been engineered to incorporate, in addition to a catalytic component, an oxygen storage capability. In operation, the oxygen storage capability absorbs excess oxygen from the exhaust stream, to allow nitrogen oxide conversion and releases oxygen to an oxygen deficient exhaust stream for hydrocarbon and carbon monoxide conversion. Ceria is typically used to provide this oxygen storage capability. To provide sufficient oxygen storage, a heavy loading of ceria is required, typically an amount at least ten or twenty times that of the precious metal loading and often as much as one hundred times greater. With current engine controls, a total weight of ceria of at least about twenty to thirty grams per liter of engine displacement is normally required or about thirty to forty grams of ceria per liter of monolith catalyst.
Short-channel, direct-coated, catalytic reactors, such as those of U.S. Pat. No. 5,051,241, incorporated herein by reference, allow for much more effective utilization of the catalytic components of the catalytic coating, thus are capable of being designed into highly efficient, compact reactors, having as little as a tenth the volume of a monolith converter sized for equivalent conversion effectiveness. A typical compact converter is forty to fifty percent the size of a conventional monolith converter for improved performance. To assure optimum conversion effectiveness in direct coated compact converters, the platinum group metals applied to the substrate represent more than 50% by weight of the coating with the remainder other materials, such as bonding agents and stabilizers. As a result, oxygen storage oxides, such as ceria, are practically limited in compact reactors to an amount less than that of the catalytic metal, typically less than twenty percent of the coating or no more than about one percent of the catalyst weight, an amount insufficient to provide an effective oxygen storage capability. As a result, customary deviations in exhaust stoichiometry hinder the ability of a stand-alone short-channel, direct-coated, catalytic reactor to meet ULEV or lower emission levels without a major improvement in engine fuel-air ratio management.
Short-channel, direct-coated reactors such as those of U.S. Pat. No. 5,051,241 can use an under layer of ceria (or alumina) prior to deposition of the platinum metal layer, but the maximum ceria content suggested to be deposited on the substrate is an amount that would create a roughly equivalent layer to that of the catalyst layer or a weight about 66% of that of the platinum. In addition, overcoating the ceria with a plating layer as taught in '241 blocks contact of the ceria with the exhaust gases rendering the ceria ineffective for oxygen storage.
The present invention makes possible the integration of an optimum oxygen storage device within a short-channel, direct-coated, catalytic reactor for the control of pollutants from an automotive internal combustion engine. This invention therefore allows an internal combustion engine to meet or even exceed California's stringent ULEV emissions requirements with available engine controls.